Fix broken ActionController#action_missing

A recent change introduced the assumption that all controller actions
are known beforehand, which is not true when using action_missing.
Don't generate a scaffold.css if no-assets
Merge pull request #9837 from kjg/no_scaffold_css_when_no_assets

Don't generate a scaffold.css when --no-assets is specified
Merge pull request #9802 from newsline/fix-broken-action-missing

Fix missing action_missing

Conflicts:
	actionpack/CHANGELOG.md